The present invention relates to a combination of a battery and a plug.
Batteries, especially lead storage batteries, invariably produce gases because of the underlying electrochemical processes during discharge and charging. The closure plugs of the cells therefore have holes through which gas exchange can occur. The ordinary plugs have a disadvantage in that they do not seal the cells completely liquid-tight, which can lead under extreme operating conditions or accidents and emergencies to leakage of very aggressive and highly corrosive electrolyte solutions, like sulfuric acid, and therefore to serious disadvantages and hazards.
German Utility Models DE-U 91 04 291 and DE-U 93 12 173 describe closure plugs for a lead storage battery in which a microporous frit or a microporous element is mounted in front of a pressure relief valve in the gas outlet direction. A disadvantage during the use of microporous elements is that the aerosols and especially sulfuric acid can clog or seal the pores of the microporous element because it creep into these. Gas permeability is extremely reduced by this and the storage battery is no longer safe.
Another disadvantage is that these elements are not liquid-repellent. For this purpose German Patent publication DE-OS 38 05 570 describes a porous inorganic substance in which the surface of the pores is coated with an internal film of silane or titanium adhesive and an external film of fluorocarbon resin. Such elements can only be produced with difficulty and are therefore very expensive.
U.S. Pat. No. 4,636,446 (LEE) discloses a stopper for a storage battery having a porous plastic body made of a spongy plastic and a spiral-shaped body provided with notches at both ends. The porous plastic body has numerous pores and is only permeable to gas. However the spongy plastic body is able to absorb water and is therefore permeable for liquids. Furthermore, the aerosols of the electrolyte creep into the pores of the spongy plastic body and block and close these pores.
French Patent FR-B-1,543,874 teaches a valve comprising a fibrous diaphragm made of highly hydrophobic fiber and an intercepting plate. This construction is not resistant to the electrolyte and aerosols of the electrolyte are able to creep into the diaphragm and block and close these pores.
The object of the present invention is therefore to improve the durability of a plug for a battery that is liquid impermeable and gas permeable.
The novel combination of a battery and a plug of the present invention is designated to solve the above discussed problems and provide significant advantages over the plugs of the prior art, as described in more detail herein.
The present invention is directed to a combination of a battery and a plug. The plug is situated in an opening of the battery and has a liquid impermeable but gas permeable membrane, being made of expanded microporous polytetrafluoroethylene (ePTFE), and an aerosol separator. The exchange of gas between the gas space and the outside of the battery takes place through the liquid impermeable but gas permeable membrane and the aerosol separator.
A plug according to the invention therefore includes an ePTFE-membrane that permits gas exchange between the gas space inside the battery and its surroundings and an aerosol separator, which separates electrolyte vapors and aerosols entrained by the electrolysis gases before they can reach the membrane. This arrangement achieves a situation in which the battery cell is degassed without the membrane being wetted by electrolyte aerosols, in which case a reduction in gas exchange would occur. Further, the novel combination of a battery and a plug ensure that the battery is protected from leakage of electrolyte in any position of the battery, because the ePTFE membrane is impermeable to liquids, especially to electrolytes like sulphuric acid. In one embodiment the membrane contains an oleophobic material. Preferably the membrane is laminated to at least one layer consisting of a support material. The support material protects and supports the membrane against mechanical loadings.
In a further embodiment the membrane has a pore size of at most 1 xcexcm. a porosity of at least 50% and a dried thickness most 5 xcexcm. These parameters ensure the high and effective gas permeability of the membrane over the lifetime of the plug. The aerosol separator is chosen from one of the following group of separators: a fiber absorber, a centrifugal separator, a cascade separator, an inertial separator, a combination of at least two of the aforementioned. Preferably the aerosol separator is a combination of an inertial separator, a centrifugal separator and a cascade separator. This construction of an aerosol separator ensures the effective separation of the aerosols in the electrolysis gas. The electrolysis gas without aerosols can reach the membrane.